Molecular sensors have been developed for selective recognition of different species on the basis of host-guest interactions making use of hydrogen bonding, electrostatic force, metal-ligand coordination, and hydrophobic and van der Waals interactions. In recent years, the development of colorimetric and fluorescent sensors of biologically active metal ions has been actively investigated because of their potential applications in life sciences, medicine, chemistry, and biotechnology. The design and synthesis of highly selective sensors for metal ions, such as mercury, lead, iron, zinc, and copper is particularly important, since these metal ions can have detrimental effects on the environment and human health.
Copper is one of the relatively small group of trace metal nutrients that are essential to sustain normal human health. The adult human body contains between 1.4-2.1 mg of copper per kilogram of body weight under normal conditions. Copper-dependent enzymes are involved in a number of physiological functions such as providing energy for biochemical reactions, transforming melanin for skin pigmentation, assisting in formation of cross-links in collagen and elastin, and thereby maintaining and repairing connective tissues. Copper in excessive amounts can be toxic and may cause oxidative stress and disorders associated with neurodegenerative diseases including Alzheimer's, Parkinson's, Menkes, Wilson's, and prion diseases. Although protein and organically bound copper appears to be less toxic, free solvated Cu2+ may be particularly damaging since it catalyzes the formation of reactive organic species (ROS), including radical and non-radical species, which can trigger oxidative damage to proteins, nucleic acids, and lipids. The common nutritional deficiencies of zinc, manganese, and other trace minerals also facilitate the accumulation of very high levels of copper.
Many colorimetric and fluorescent chemosensors have been reported for sensing metal ions, with absorbance and emission in the visible region. However, the visible region spectrum suffers from various drawbacks such as lack of penetration into the test sample due to absorption and scattering of light. Further, the presence of autofluorescence generated from the chromophores and macromolecules present in the analytic samples impede the use of fluorescent chemosensors. These limitations can be overcome by the use of NIR radiation as they can penetrate the sample much deeper due to limited absorption and scattering. Therefore, there exists a need to develop molecular probes with NIR (700-1000 nm) optical responses for detecting metal ions.